Mono-crystalline sapphire is characterized by an extraordinary hardness and stretch resistance. Sapphire plates, generally referred to as sapphire glass, are therefore frequently used in the watch industry. It is desirable to broaden the scope of application of sapphire plates and to use them as transparent covers for display devices of electronic devices. It is generally conceivable to equip electronic displays, for example of tablet computers, mobile phones and smartphones, with sapphire plates.
However, due to its crystalline properties, in particular the mono-crystalline structure, sapphire is relatively brittle. Relatively large and thin sapphire plates, as would be required for example as covers for smartphones of tablet computers, therefore have a break resistance during bending and heavy impact, which is insufficient for practical application.
For production reasons, the surface of sapphire plates typically has defects and/or micro-cracks. Such micro-cracks can propagate relatively fast in the plate material during bending or heavy impact and can ultimately lead to breaking of the sapphire plate.
In the publication of C. M. Liu, J. C. Chen, L. J. Hu, S. P. Lin “The effect of annealing, precipitation-strengthening, and compressive coating processes on sapphire strength”, Material Science and Engineering A 420 (2006) 212-219, the hardening of sapphire by means of tempering is described, among others, during which the sapphire plates are heated up to temperatures of at least 1500° C. for a relatively long time, i.e. approximately 24 hours.
Surface defects can be removed. However, the method takes a relatively long time as well as being cost-intensive and energy-intensive. Furthermore, the coating of sapphire substrates with silicon nitride is described as an anti-reflection coating. Due to the subsequent thermal hardening process, to which the coating is subjected for at least an hour at a temperature of 1500° C., the applied silicon nitride layer can crystallize.
Since silicon nitride has a higher density in the crystalline state than in the amorphous state, a compressive stress can be generated on the sapphire surface. As a result, the micro-cracks or defects present on the surface bordering with the applied silicon nitride layer can virtually be pulled together and therefore be minimized. The sapphire plate can therefore be tensioned or pre-tensioned by the coating.
The refractive index of a silicon nitride layer is typically above 1.9. The refraction index of pure silicon nitride is even 1.98. In contrast, mono-crystalline sapphire has an average refraction index of 1.74 in the range of the visible spectrum. The silicon nitride layer alters the optical properties of a sapphire plate.
Consequently, silicon nitride is highly reflective and has a reflectance on sapphire of more than 30%, so that it can generally not be used for coating of cover plates of display devices.